Ceramic materials that deform plastically at room temperature to demonstrate a measure of ductility have long been sought. The scientific literature has described some heavily microcracked bodies purported to be highly compliant. The mechanical strengths exhibited by those bodies have generally been quite low, however, as would be expected. Studies of fiber reinforced glasses and glass-ceramics have also shown promise, but they again, in some sense, rely upon microcracking of the matrix and the fiber-matrix interface to obtain their good fracture properties.
Recently, the following two types of zirconia-based ceramic materials have been discussed in the literature as demonstrating limited pseudo-plastic deformation:
The first type is based upon observations of non-linear stress-strain behavior in certain transformation toughened, partially-stabilized ZrO.sub.2 bodies. To illustrate:
M. V. Swain in "Inelastic Deformation of Mg-PSZ and Its Significance for Strength-Toughness Relationship of Zirconia Toughened Ceramics," Acta Metall., 33, No. 11, pages 2083-2091, 1985, described non-linear stress-strain behavior in large grain size (50-100 micron) magnesia partially-stabilized ZrO.sub.2.
M V. Swain and L. R. F. Rose in "Strength Limitations of Transformation-Toughened Zirconia Alloys," Journal of the American Ceramic Society, 69, No. 7, pages 511-518, July 1986, described magnesia partially-stabilized zirconia exhibiting a significant amount of pseudo-plasticity, apparently derived from microcracking and phase transformation of zirconia associated with that microcracking. The mechanical strength demonstrated by those materials was no more than about 100 Kpsi.
K. Tsukuma and M. Shimada in "Strength, Fracture Toughness and Vickers Hardness of CeO.sub.2 -Stabilized Tetragonal ZrO.sub.2 Polycrystals (Ce-TZP)," Journal of Materials Science, 20, 1178-1184 (1985) disclosed CeO.sub.2 -stabilized zirconia bodies manifesting some pseudo-plasticity, again as a result of microcracking and attendant phase transformation of ZrO.sub.2. Nevertheless, the contribution of each to the development of pseudo-plasticity, i.e., the microcracking and the phase transformation, was not defined. In any event, the CeO.sub.2 -containing bodies exhibiting pseudo-plasticity demonstrated mechanical strengths less than 75 Kpsi.
P. E. Reyes-Morell, J. Cherng, and I-Wei Chen in "Transformation Plasticity of CeO.sub.2 -Stabilized Tetragonal Polycrystals: II, Pseudoelasticity and Shape Memory Effect," Journal of the American Ceramic Society, 71, (8), pages 648-657, 1988, described stress-strain behavior of 12 mole percent CeO.sub.2 -TZP under uniaxial compression and hydraulic compression and shape recovery by heating. They also provided a rather detailed theoretical analysis of the mechanism underlying such deformation.
The second type of zirconia-based materials exhibiting pseudo-plastic deformation has involved a low temperature, water reaction mechanism. To illustrate:
U.S. Pat. No. 4,767,730 disclosed ZrO.sub.2 -containing articles which plastically deformed through a phase transformation taking place between the tetragonal and monoclinic phases of ZrO.sub.2 by maintaining the articles at a given temperature and imposing a given stress thereon. The original configuration of the articles can be restored by again holding the articles at a given temperature greater than that used to deform the article and applying a specified stress thereupon. The patent termed that phenomenon "shape memory," thereby correlating the effect with that observed in certain metal alloys.
The patent attributed the mechanism underlying the phenomenon as constituting a martensite transformation from tetragonal ZrO.sub.2 in a metastable state to monoclinic ZrO.sub.2. Inasmuch as the phase transformation between the tetragonal phase and the monoclinic phase is induced by an applied stress when an appropriate stress is imposed thereon, the article is plastically deformed after normal elastic deformation without being broken. The patent also observed that the desired activating phase transformation is accelerated when the stress is applied in a water-containing environment, and that the rate of acceleration can be increased by raising the water content in the atmosphere. A review of the patent seems to indicate that practical rates of plastic deformation were achieved only at temperatures of about 200.degree.-300.degree. C. in the presence of water vapor.
The working examples provided in the patent utilized bodies consisting of a ZrO.sub.2 -Y.sub.2 O.sub.3 solid solution (94.6% ZrO.sub.2 -5.4% Y.sub.2 O.sub.3 by weight), which bodies were subjected to a stress of about 50-3000 MPa at a temperature of about 0.degree.-400.degree. C. to effect the plastic deformation.
Visual microscopic examination and electron microscope studies of the articles resulting from the disclosure of that patent have indicated the presence of significant microcracking due to a reaction taking place with water vapor. In essence then, the pseudo-plasticity demonstrated by the articles of the patent is founded upon the same mechanism as that underlying the two types of products described in the scientific literature which were discussed above, such that the articles of the patent are subject to the same problems and drawbacks.
I. Nettleship and R. Stevens in "Tetragonal Zirconia Polycrystal (TZP)-A Review," International Journal of High Technology Ceramics, 3, pages 1-32, 1987, provide an extensive discussion of the low temperature (150.degree.-350.degree. C.) water vapor aging or corrosion phenomenon, such as is disclosed in U.S. Pat. No. 4,767,730 above, as well as a review of the pseudo-plasticity evidenced by CeO.sub.2 -stabilized TZP. The authors focus on the water vapor aging as a detrimental phenomenon and also state that, despite the high toughness values displayed by the CeO.sub.2 -stabilized TZP materials, the mechanical strengths exhibited by them are so low that they "will require improvement before Ce-TZP presents a serious challenge to Y-TZP."
Therefore, the primary objective of the present invention was to produce partially-stabilized ZrO.sub.2 /HfO.sub.2 ceramic articles demonstrating pseudo-plasticity, but wherein the mechanism effecting that phenomenon does not involve the occurrence of substantial microcracking, whether occurring during transformation of the tetragonal phase to the monoclinic phase or as a result of water vapor aging, such that the articles exhibit high mechanical strength.
Another objective of the present invention was to produce such ceramic articles exhibiting pseudo-plasticity coupled with high toughness.